Evaluation of the RF field uniformity of a double‐tuned ³¹P/¹H birdcage RF coil for spin‐echo MRI/MRS of the diabetic foot

Abstract

Purpose: To evaluate the B1 field uniformity of a doubletuned birdcage coil designed for 31 P/ 1 H MRI/MRS spinecho (SE) imaging of the metatarsal head region of the foot in neuropathic diabetic patients. Materials and Methods:A low-pass double-tuned 31 P/ 1 H RF birdcage coil was constructed to fit over the adult forefoot. Flip angle (FA) maps were created from B1 data acquired at the 3T 31 P (four normal subjects) and 1 H (five normal subjects) frequencies. T2-weighted (T2-W) 1 H images, 31 P rapid acquisition with relaxation enhancement (RARE) images, and composite SE pulse CSI data were acquired to demonstrate the uniformity of the resulting images and data. Results:The means and standard deviations (SDs) of the range of FAs across the feet of the volunteer subjects indicated good uniformity (the maximum coefficients of variation (CVs) for all of the 31 P and 1 H FA maps were 7.6% and 7.3%, respectively). The FA values across the metatarsal head region indicated a maximum signal intensity variation of Ϯ3% in a RARE image acquired using an echo train length of 32. IN DIABETES, functional abnormalities of the microvasculature in the distal regions of the lower extremities result in polyneuropathy, which causes a loss of sensation, vasodilation, and motor function in the muscle (1). These complications have especially profound effects in the feet. Sensory, motor, and autonomic neuropathies in the distal regions of the extremities predispose many diabetics to traumatic insults leading to superficial ulcerations and osteomyelitis (2,3). These pathologies typically occur at the metatarsal head region of the forefoot. The energy metabolism of the muscles in this region may be a measure of the degree of ischemia and the viability of the affected tissues. Thus, a phosphorus-31/proton ( 31 P/ 1 H) MRI/MRS examination of the forefoot of diabetic patients could be valuable for assessing the severity of polyneuropathy, monitoring therapeutic intervention, and planning surgical procedures. Conclusion:Recently introduced spin-echo (SE) methods offer advantages for 31 P imaging and spectroscopy. Direct imaging of the 31 P metabolites can be accomplished with substantially reduced scan times and improved spatial resolution compared to chemical shift imaging (CSI) MRS techniques using the rapid acquisition with relaxation enhancement (RARE) (4,5) pulse sequence. Also, an SE spectroscopic technique that uses a composite excitation and refocusing pulse to reduce the echo time (TE) has been introduced (6). This technique may have advantages for 31 P spectroscopy of human skeletal muscle. The loss of early data points in the free induction decay (FID) of pulse-and-acquire sequences results in reduced SNR and baseline distortion in 31 P spectroscopy (7). It is also useful to combine SE 1 H imaging with 31 P MRS to acquire SE T2-weighted (T2-W) and proton density (PD)-weighted images (8) for anatomical registration and identification of pathology (2). However, the signal produced by SE sequences is much more susceptible to spatial ...

Treating diabetic ulcers

“…When energy demand is greater than mitochondrial capacity, or in the presence of ischemia, the concentration of PCr drops and the phosphate moiety for the conversion of ADP to ATP is depleted, resulting in increased Pi concentration. Recent findings from our unit demonstrate that muscle metabolism and tissue hypoxia (as evidenced with an increased Pi/PCr ratio) are present in the foot muscles of diabetic patients with or without neuropathy in the absence of PAD (Figure 4) [47][48][49][50][51]. Further investigation into these novel techniques may help identify screening and treatment strategies to better address DFU.…”

The Feasibility of Measuring Phosphocreatine Recovery Kinetics in Muscle Using a Single-shot 31P RARE MRI Sequence

“…To determine the region of sensitivity of the 31 P surface coil a map of the RF (B1) field was generated, as previously described (17, 18), by placing one of the flat end surfaces of a 16 cm diameter × 25 cm long cylindrical phantom containing a solution of 50 mM inorganic phosphate (Pi) onto the 7.5 cm circular 31 P surface coil. The spatial resolution of the RF field map was chosen to match that of the PCr RARE images.…”

“…A double-tuned ( 31 P/ 1 H) low-pass quadrature birdcage RF coil with a diameter of 12 cm and a length of 12 cm was used for all image acquisitions for this study (17). All imaging was performed in an axial plane located 5 cm distal to the elbow joint.…”

Simultaneous acquisition of phosphocreatine and inorganic phosphate images for Pi:PCr ratio mapping using a RARE sequence with chemically selective interleaving

“…A double-tuned ( 31 P/ 1 H) low-pass quadrature birdcage radio frequency (RF) coil consisting of 8 struts with a diameter of 12 cm and a length of 12 cm was used for all imaging (13). …”